NQR is a magnetic resonance technique, closely related to Nuclear Magnetic Resonance (NMR), suitable for detection and/or analysis of bulk materials that contain a quadrupolar nucleus. Examples of such materials are nitrogen-containing explosives such as RDX, TNT and PETN and chlorine-containing narcotics such as heroin and cocaine. An advantage of NQR over NMR is that there is no requirement for a strong direct current (DC) magnetic field. This removes the need for a large, expensive, typically super conducting, electro-magnet.
Atomic nuclei with a spin quantum number of greater than ½ and having non-spherical electric charge distributions possess electric quadrupole moments. Quadrupole resonance arises from the interaction of the nuclear quadrupole moment of the nucleus with the local applied electric field gradients produced by the surrounding atomic environment.
NQR analysis for a given material involves the irradiation of a sample that has been placed in a test volume with a pulsed RF magnetic field. The frequency of the applied field used must be at or very close to one of the nuclear quadrupole resonance lines of the material under analysis. These frequencies are unique to individual materials and therefore allow for very specific identification of a material under analysis.
NQR has applications in a number of fields including security screening for the detection of drugs or explosives in bags, cargo etc.; landmine detection; pharmaceutical processing; pharmaceutical and chemical production quality control; and strain gauge measurements.
In the past, baggage screening systems have employed NQR to identify suspect bags that may contain contraband. U.S. Pat. Nos. 5,206,592 and 5,233,300 describe methods and systems for detecting nitrogenous explosives and narcotics employing NQR techniques. These techniques, however, have lower signal to noise ratio causing problems due to interference from external signals, thereby masking the signal.
Typically these systems will sequentially search for two or more explosive materials using pulse sequences at multiple frequencies corresponding to the NQR frequencies of the materials being searched. A high frequency resonant circuit is used to generate the magnetic field required to excite the NQR signal and subsequently to detect the generated magnetic response. These systems are built with a high quality factor (Q), which is important for two reasons. Firstly, the signal to noise ratio of any signal detected is directly related to Q1/2. Secondly, the power required to generate a suitable excitation signal is directly related to the Q factor.
Essentially, resonator probes previously used in this manner have been single turn solenoids made from sheet copper with a gap in them, which are bridged with capacitors. The capacitors are selected such that the resonant frequency of the coil/capacitor combination is the same as the NQR frequency of the material under investigation.
There are several limitations inherent to solenoidal coil design. Solenoidal coils are good magnetic structures for generating homogeneous RF magnetic fields. However, half of the magnetic flux generated by such a structure is outside the structure. Owing to the nature of magnetic fields, unless this external flux is constrained by a shield, it will be measurable at considerable distances from the solenoid. Although the addition of electromagnetic shielding considerably reduces this, inadequate shielding leads to two primary problems. Firstly, magnetic fields generated within the system escape at sufficient levels such that the fields generated interfere with other electrical equipment. Secondly, electromagnetic interference generated outside the equipment can be picked up by the equipment and can interfere with the highly sensitive NQR measurement.
To allow for effective performance of the probe coil, the shield must be suitably spaced from the resonator probe. Additional magnetic probes within the same shield must be at considerable distances from the first probe and each other. In effect, this means that it is generally practical to put only one resonator probe in a shield. The shield used must also be significantly larger than the resonator probe it is shielding.
U.S. Pat. Nos. 5,592,083, 6,194,898 and 6,291,994 disclose a NQR based contraband detection system with electromagnetic shielding to mitigate low signal to noise ratio and the presence of external interference signals. In these systems, the RF coil is comprised of a hollow rectangular tube of thin sheet conductive material formed on a thin-walled rectangular insulator. The shield is a rectangular conductor sleeve, comprised of a copper or any other highly conductive material, and encloses the probe coil. U.S. Pat. No. 6,522,135 discloses a rectangular resonator probe employed to detect NQR in the sample.
Such systems will have a single inspection resonator probe that scans at the frequency corresponding to the first target material. If it is necessary to investigate a second material the resonator probe is then retuned by switching its resonant frequency to that of the second target material. The frequency of operation is typically changed by switching the amount of capacitance in and out using relays or mechanical actuators. In addition to this coarse frequency adjustment it may be necessary to fine tune for each frequency range depending on the electrical properties of the item (bags or packages) that is being examined. Conductive or high permeability, materials in particular, will alter the inductance of the resonator coil and, therefore, its tune frequency. The de-tuning effect of the bag on the resonator probe will differ from bag to bag depending on their contents and construction. As a result the probe must be retuned to a new NQR frequency. U.S. Pat. No. 5,457,385 discloses a NQR based detection system having an array of excitation devices (RF coils) tuned at different frequencies for determining the presence of selected nuclei in an article. These systems, however, require costly tuning relays for tuning the system to one or more requisite NQR frequencies.
Since much of the equipment that uses this technology is for use in airports, where space is at a premium, it is desirable to build systems as compactly as possible. Furthermore, this technology complements existing airport screening equipment, for example X-ray machines and Computed Tomography (CT) machines. U.S. Pat. Nos. 5,168,224 and 5,642,393 disclose an inspection system for detecting a specific substance in an article using an X-ray inspection apparatus in conjunction with NQR measurement equipment. The resulting solenoidal resonator probe design, however, does not lend itself to close integration with parts of other existing sensor systems.
At present, solenoidal resonator probes have been used because it is practical to use only one inspection resonator probe even when systems are investigating for the presence of more than one material at more than one frequency. As described above, this is achieved by using either relays or mechanical actuators to switch between varying capacitance values. This is disadvantageous because the relays and mechanical actuators used must have very low contact resistance to minimize the resistive losses within the resonant circuit and therefore maximize the Q-factor. Even the use of high quality components is not completely effective in countering resistive loss. Typically, the lower frequency ranges suffer the most from additional loss because the lower the frequency the more additional capacitance that has to be switched in. Additionally, it is often the lower frequencies that have lowest NQR sensitivity due to the lower induced signal voltages picked up at lower frequency.
Another disadvantage in switching the resonant frequency of a single resonator probe is that the relays or mechanical actuators described above are very expensive. Typically, they are one of the most expensive components of the resonator probe. Additionally, since relays and actuators are predominantly mechanical devices they are also one of the most likely components within the resonator probe to suffer from mechanical failure. Often multiple relays are connected in parallel to yield the desired low contact resistance. It becomes difficult to diagnose the problem if any one of these devices fails.
European Patent Application No. 1,253,433 discloses an extended toroidal design for the resonator probe. The resonator probe is designed to improve sensitivity to signals generated within a sample volume while improving insensitivity to background noise. It also discloses a tuning vane provided within the hollow central portion of the resonator probe for varying inductance of the probe coil for the purpose of tuning. This design does not, however, lend to efficient integration of additional resonator probes or other existing screening systems.
Thus, what is needed is a compact resonator probe that can be placed in proximity to shielding devices, additional resonators probes, and other components of an article screening system. What is also needed is a resonator probe in which the number of relays or mechanical actuators employed is reduced or eliminated.